On Mar 27, 2008, at 2:12 PM, Cooke Kelsey wrote:
> Here's the answers to your questions:
>> Q: What do you mean with (rest ..) ? This doesn't look like my table.
>> A: Your table says, "(insert-everywhere/in-all-words s (rest
> low)," which means I would rewrite the whole function in the column?
I want to know what the result of (insert-everywhere/in-all-words s
(rest low)) would be when low = ...
(Looking more closely, your second row says you did the right thing.)
> [This isn't quite right! 'AT is NOT the representation of the
> English word "at".]
>> A: I was trying to use shorthand from Intermezzo 2:
> '(a b c) This short-hand is an abbreviation for (list 'a 'b 'c)
Then what is '(AT)? Hint: you used ' improperly, and that is common
for beginners. I recommend you stick to (list 'a 'b 'c) and (list
(list 'a) (list 't)) and so on.
> Q: Why not take one step from row 1, which uses a list with one
> word. What would be the next case?
>> A: Um, two words. I have no idea why I did that.
Make up a simpler example. Simple examples are good.
> Q: The second leap went wrong. Why? Is it possible that you
> JUMPED? Why don't you turn the above examples into examples for
> this function. And *please* do provide a contract, a purpose
> statement.
>> A: OK, this is where my big problem is. I'm not sure what you mean
> by JUMPED. Can you be more specific? I realize this is some kind
> of clue but I am too slow to get it.
JUMPED means you wrote the helper function w/o contract, purpose
statement, template, examples.
> Here's a full design recipe:
>> ; Purpose: to insert a symbol between every letter of a single word
> ; Contract: symbol word --> list of words
> ; Example: (define (insert-in-single-word X AT). Result should be
> (XAT AXT ATX)
(0) The example is _conceptually_ correct (meaning at the information
level).
(1) At this point it becomes critical to spell out the (list ...)
stuff, i.e., to turn the example into full-fledged Scheme data. Small
step.
(2) I propose you make a second example again.
> ; Template:
> ; (define (insert-in-single-word s word)
> ; (cond
> ; [(empty? (rest word)) empty]
> ; [else (append (list (first word) s (rest word)) (insert-in-
> single-word s (rest word)))]))
This is NOT a template.
And before you complete the template build a table just like the one
above for the recursive case.
-- Matthias
>> Thanks very much.
>> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>> On Mar 27, 2008, at 1:20 PM, Cooke Kelsey wrote:
>> Dear Dr. Felleisen,
>> thanks so much for responding! Your suggestions really helped me
>> to think about the problem.
>>>> Here's a table of examples, as you suggested:
>>>> ;;Examples
>> ; s | low |(first..)|(rest..) | result
>> Q: What do you mean with (rest ..) ? This doesn't look like my table.
>>>> ;--------------------------------------------------------------------
>> ------------------------------------
>> ; X | '(AT) |'(AT) |empty | '((XAT) (AXT) (ATX))
>>> [This isn't quite right! 'AT is NOT the representation of the
> English word "at".]
>>>> ; X | '((AB) (CD) (EF))|'(AB) |'((CD) (EF))| '((XAB) (AXB)
>> (ABX) (XCD) (CXD) (CDX) (XEF) (EXF) (EFX))
>> This second row looks hyper-complicated.
>> Q: Why not take one step from row 1, which uses a list with one
> word. What would be the next case?
>>>>>> Here's an attempt to "go from the first columns to the last":
>>>> ; Definitions:
>> (define (insert-everywhere/in-all-words s low)
>> (cond
>> [(empty? (rest low)) empty]
>> [else (append (insert-in-single-word s (first low)) (insert-
>> everywhere/in-all-words s (rest low)))]))
>> Good the first leap went fine.
>>>> (define (insert-in-single-word s word)
>> (cond
>> [(empty? (rest word)) empty]
>> [else (append (list (first word) s (rest word)) (insert-in-
>> single-word s (rest word)))]))
>> The second leap went wrong. Why?
>> Is it possible that you JUMPED? Why don't you turn the above
> examples into examples for this function. And *please* do provide a
> contract, a purpose statement.
>> -- Matthias
>>>>>>> Unfortunately, the list of words gets shorter and shorter (sATX,
>> sTX, sX). I tried to think of a way to add a recursive "prefix"
>> that gets longer, but each time the function cycles the first
>> letters seem to disappear into the void...
>>>> Matthias Felleisen <matthias at ccs.neu.edu> wrote:
>>>> On Mar 25, 2008, at 1:48 PM, Cooke Kelsey wrote:
>>>>>> I am trying to learn HtDP by myself, and like Dave Yrueta I am
>>> stumped by 12.4.2.
>>>> [Oops I forgot to reply to his last message.]
>>>>>>> ; Purpose: to insert a symbol between every letter of every word,
>>> e.g. X, AT-->XAT, AXT, ATX.
>>>> Please reformulate the example in terms of symbols and lists.
>>>>>>> ;Function Template:
>>> (define (insert-everywhere/in-all-words s low)
>>> (cond
>>> [(empty? (rest low))...]
>>> [else ...(first low)...insert-everywhere/in-all-words s (rest
>>> low)]))
>>> Note:
>>> It is suggested to use "append", which is easy one or two times---
>>> (append x word) or (append (append (first word) x) (rest
>>> word))----but I can't see how to use this recursively for an
>>> arbitrary number of letters and words.
>>>> So far so good. Let's try what HtDP/2e will introduce:
>>>> Please make a table of the following shape:
>>>> s | low | (first low) | (insert-everywhere/in-all-words
>> s (rest low) | expected result for (insert-... s low)
>> ---------------------------------------------------------------------
>> -----------------------------------
>> X | AT | ? | use the purpose statement to
>> determine this | (list `XAT' `AXT' `ATX')
>>>>>> As I said, you need to reformulate this in terms of symbols and
>> lists. If it doesn't jump out at you how to go from the four
>> columns to the last one, then make a couple of more examples. I am
>> pretty sure it will. Here is the rule of thumb:
>>>> --> you know of a built-in function/primitive that does the work
>> --> you don't. in this case, you make a wish: a purpose statement
>> and a contract for a helper function that does it for you.
>>>> You may also have to combine a primitives with functions.
>>>> Please report back -- Matthias
>>>>>>>>>>>>>>>>>>>> The fact that the "hint" refers to the keyword list, which the
>>> book hasn't covered yet, makes me wonder if it's even possible to
>>> answer this problem, given what we know so far...
>>>>>> Thanks very much,
>>>>>> Cooke
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